1. Field of the Invention
This invention relates to an infrared absorption enhanced spectroscopic apparatus. More particularly, this invention relates to an infrared absorption enhanced spectroscopic apparatus in which a thin layer of a sample to be analyzed is placed in the gap between a reflecting face of a high refractive-index prism that also has an entrance face and an exit face and a metal as solid plasma comprises free electrons.
2. Description of the Related Art
Materials such as metals that permit electrons to move freely may be regarded as solid plasmas. Surface electromagnetic waves (surface plasmons) can exist in the neighborhood of the surfaces of such solid plasmas. Photoexcitation of surface electromagnetic waves can be achieved by using either a high refractive-index prism or a diffraction grating formed on the surface of a metal.
FIG. 5 is a sketch illustrating the concept of surface electromagnetic wave excitation proposed by Andreas Otto in "Excitation of Nonradiative Surface Plasma Waves in Silver by the Method of Frustrated Total Reflection", Zeitschrift fur Physik, 216, pp. 398-410 (1968). Shown by 1 is a prism having a high refractive index n.sub.p that is placed on top of a metal 3 having a dielectric constant of .alpha.(.omega.) with a gap 2 of a dielectric constant of .alpha.(.omega.) being formed between the two elements; .omega. is the angular frequency of light and dielectric constants .theta. and .epsilon. are each a function of .omega.. Incident light 4 is reflected totally by the bottom surface of the prism 1. The reflected light 5 travels through the prism. The wave number k.sub.p (the inverse of the wavelength) of the light travelling through the prism is expressed by .eta..sub.p .multidot..omega./c (c is the velocity of the light). If k.sub.p SIN.theta., or the component of k.sub.p in the x-direction is equal to the wave number of surface electromative waves to be excited, k.sub.x, then surface electromagnetic waves 6 are excited to occur on the surface of the metal 3.
The wave number, k.sub.x, of surface electromagnetic waves 6 can be correlated to the angular frequency, .omega., of the light in terms of dispersion by the following equation: ##EQU1##
With electromagnetic waves of comparatively low frequencies such as infrared light, the absolute value of the dielectric constant of a metal .vertline..epsilon..vertline. is far greater than the absolute values of the dielectric constants of common dielectrics. Therefore, equation (1) can be approximated as follows: EQU k.sub.x =.omega./c.sqroot..eta. (2)
This indicates that surface electromagnetic waves of low frequencies behave like photons that are guided through a gap by a good electric conductor.
By the principle described above, infrared light can be transmitted through a thin layer of a sample on the surface of a metal in a direction parallel to the surface of the thin layer, and an improved sensitivity of analysis can be obtained if the infrared light is permitted to pass through the thin-film sample over an increased distance.
Surface electromagnetic waves have been studied for many years and methods of exciting surface electromagnetic waves using high refractive-index prisms have been proposed not only by Otto, supra, but also by Erwin Kretschmann in "Die Bestimmung optischer Konstanten von Metallen durch Anregung von Oberflachenplasmaschwingungen"chenplasmaschwingungen" Z. Physik, 241, pp. 313-324 (1971).
Spectroscopy with surface electromagnetic waves in the strict sense of the term would typically involve the use of two spaced prisms for measuring the ratio of the intensity of light propagating through the surfaces of the gap to the intensity of incident light. However, it has been verified that surface electromagnetic waves can also be excited by using a single prism in the method of Otto or Kretschmann. It is possible to increase the sensitivity of analysis by enhancing the absorption of infrared light by a thin layer of sample deposited on the surface of a metal, which infrared light has been passed in a direction parallel to the surface of the thin layer.
Various attempts have been made in order to realize this possibility. See, for example, A. Hatta, T. Ohshima and W. Suetaka, "Observation of the Enhanced Infrared Absorption of p-Nitrobenzoate on Ag Island Films with an ATR Technique", Appl. Phys., A29, pp. 71-75 (1982) and A. Hatta, Y. Suzuki and W. Suetaka, "Infrared Absorption Enhancement of Monolayer Species on Thin Evaporated Ag Films by Use of a Kretschmann Configuration: Evidence of Two Types of Enhanced Surface Electric Fields", Appl. Phys., A35, pp. 135-140 (1984). Suetaka et al. observed enhanced infrared absorption by thin films deposited on evaporated silver films on the flat surface of a hemicylindrical germanium prism. Ishida et al., Yuichi Ishino and Hatsuo Ishida, in "Grazing Angle Metal-Overlayer Infrared ATR Spectroscopy", Applied Spectroscopy, Vol. 42, No. 7, pp. 1296-1302 (1988), described the use of a germanium prism having both a flat entrance face and a flat exit face so as to provide an incident angle of 75.degree. and observed that infrared absorption could be enhanced by evaporating silver on the surface of a thin layer of sample that was deposited on the reflecting face of the prism.
In all of these methods, metal layers are formed by evaporation. Special equipment such as a vapor deposition system are necessary if one wants to apply those methods to routine analyses. Further, the two methods of Suetaka et al. enhance the infrared absorption by using island Ag particles. Even if the involvement of surface electromagnetic waves is taken into account, the need to use a particular metallic material (Ag) and deposit it in the form of island films is a very strict limitation on the use of those methods by ordinary analysts.
Another paper that may be cited as a general reference that describes the background of the present invention with respect to the use of surface electromagnetic waves in spectroscopy is Robert J. Bell, R. N. Alexander, Jr., C. A. Ward and I. L. Tyler, Introductory Theory For Surface Electromagnetic Wave Spectroscopy", Surface Science, 48, pp. 253-287 (1975).